2 Answers
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In manual mode (M), it indicates how far you are from what the camera thinks should be the right value. It is the camera's guess (depending on the metering mode, see other questions in metering).

In all other modes, it is called exposure-compensation (EC) and is you telling the camera to adjust its measurement to something brighter or darker.

In general the camera meters so that the scene is on average of average brightness. By doing over the metered value, you are telling the camera you want something brighter and by going under the value you are telling you want something darker.

The metering scale is measured in EV (Exposure Value) where each full-step represent a doubling of brightness. So +1 is twice as bright, +2 is 4 times as bright and -1 is half as bright, etc.

"Understanding Exposure" by Bryan Peterson is an excellent read for understanding what your cameras light meter is actually representing and what situations it will lie to you. Tons of good situational examples too; snow, forest, backlit etc.

Does Peterson actually say that the meter lies to you? I think that's a non-helpful way to explain. The meter isn't smart enough to do anything of the sort, and I think it's counter-helpful to use explanations that pretend it is. It's not lying, or even wrong -- it's just doing the one thing it knows how to do. (With the exception of matrix metering, which can't lie either, but can be wrong if it mismatches the scene.)
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mattdmDec 3 '10 at 0:23

lol, okay. So pedantically speaking the meter isn't "lying" to you. But say you (like the OP) are a beginner photographer taking a photo of snow or bright sand. The meter will tell you the shot will be nicely exposed when it will actually be horribly underexposed. WTH! That lying camera!! You told me it would be perfectly exposed!! In the book Peterson explains why this is and what the camera/meter is actually measuring.
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bigwebguyDec 4 '10 at 17:29